A large number of different devices have been used for measuring pressure. These devices include, in general, a mechanical element that undergoes a displacement in response to an applied pressure, and a device that senses the magnitude of this displacement. The relationship between the displacement and the applied pressure is a function of the elastic properties of the material from which the mechanical element is made. Errors are introduced into the measurement as a result of physical properties inherent in mechanical devices, including the coefficient of thermal expansion, hysteresis of the material, and the thermo-elastic modulus. Other factors relating to the mechanical sensor include the strength of the sensor, its susceptibility to damage by the environment, and ease of fabrication. Additionally, measurement of high pressures, up to 10,000 pounds per square inch, requires sensors with high strengths. Various devices are commonly used to sense the mechanical displacement in a pressure transducer, including devices utilizing changes in resistance, capacitance, magnetic reluctance, and piezo-electric effects.
Pressure-sensing devices using the change in capacitance between two, opposed, electrically-conductive materials have been proposed. See for example U.S. Pat. No. 3,195,028, for a Capacitance Pressure Gage. Capacitive pressure transducers are typically used to control the frequency of an oscillator, which is advantageous for converting the output to a digital form for further processing. Disadvantages of capacitive pressure gages include the requirement of insulating the capacitive plates from one another, inaccuracies resulting from parasitic capacitances contributed by interconnecting cables and driving circuitry, and errors resulting from hysterisis and elastic creep in the materials from which the transducer is fabricated.